But how're you gonna gloss over the fact that the format is not specified, making it impossible to convert an <img> to a texture or a texture back to bytes or an <img>?
I'll agree that it's a less-than-ideal situation, but I'm not sure that there's a lot of utility in ensure textures can always be represented in <img> tags. The currently implemented S3TC textures must be manually unpacked and rendered to a canvas or data url before they can be shown in the DOM, but I've only once seen anybody bother. As long as the hardware can unpack it properly I have a hard time seeing why we shouldn't consider exposing it to give us wider hardware coverage. We don't have a whole lot of alternatives if we want compressed texture support, sadly.